Very early in the evolution of the internal combustion engine, it was discovered that noise emitted by the engine could be reduced by resonant sound muffling devices (muffler). In general, such mufflers comprised acoustic filters, which generally employ interference principles to cancel sound. Typically, the sound waves generated by the engine are broken into two parts. Each part follows a separate path having a different effective length. Ultimately, before leaving the muffler, the waves meet at a point where they are 180.degree. out of phase and are thus cancelled. For example, a quarter wavelength resonant cavity, comprising a chamber closed at one end and open at the other, is known as a Helmholz cavity. Sounds entering the open end of the chamber pass through and are reflected back to the open end of the chamber with a phase delay of one-half of a wave length, i.e., 180.degree.. Such cavities can be created employing a matrix of baffles. Conventional automotive mufflers pass exhaust gases over a matrix of baffles, which together define a plurality of such tuned cavities, to cancel the particular sound frequencies to which the chambers are tuned.
Such acoustic mufflers are disadvantageous in a number of respects. Such mufflers tend to create an undesirable back pressure on the engine, are commonly bulky, require a relatively high degree of maintenance, and are limited in effective frequency range. For example, the effectiveness of conventional resonant sound mufflers is limited for sound frequencies below about 100 Hz; sound occurring below that frequency is not efficiently diminished and propagates to the ambient atmosphere as unwanted noise.
However, electronic mufflers which are capable of diminishing low frequency noise are also known. Electronic mufflers eliminate unwanted engine noise using destructive interference (the principle of superposition). Cancellation is achieved by propagating "anti-noise", an acoustic signal having the same spectral characteristics as the unwanted noise but 180 degrees out of phase. The anti-noise interacts with the unwanted engine noise and results in cancellation. An electronic muffler system typically includes a sound generator (e.g. speaker or speakers) and a controller. The sound generator produces anti-noise in accordance with drive signals from the controller. The controller drives the sound generator according to signals representative of the noise field (exhaust noise) to be canceled.
More specifically, the residual noise (i.e., the noise remaining after superposition) is sensed, typically by a microphone disposed to sense the exhaust noise, and a signal indicative of the residual noise is provided to the controller. In many systems, a signal indicative of the periodicity of the exhaust noise (e.g. a tachometer signal indicative of engine speed) is also provided to the controller. The controller drives the actuator accordingly. Examples of electronic muffler systems are described in: U.S. Pat. No. 4,527,282, issued Jul. 2, 1985, to Chaplin, et al.; U.S. Pat. No. 5,097,923, issued Mar. 24, 1992, to Ziegler, et al.; French Patent 1,190,317, issued March, 1959, to Scherrer; G. B. B. Chaplin, The Cancellation of Repetitive Noise and Vibration, INTER-NOISE 80, December 8-10, (1980); G. B. B. Chaplin, Anti-Noise--The Essex Breakthrough, CHARTERED MECHANICAL ENGINEER, page 41 (Jan. 1983); and I. Brown, The Application of Simple Source Theory to Active Noise Control, PROCEEDINGS OF THE INSTITUTE OF ACOUSTICS (1985).